CONTINENTAL DRIFT
The idea that the continents drift relative to each other was one that few hurried to accept when the German geologist Alfred Wegener (1880-1930) proposed it in 1912. Yet recent research has revealed a variety of evidence showing that the lithosphere is indeed segmented into about a dozen or so major plates of different sizes. Floating on the earth's mantle, they move slowly, carrying the continents with them at an average rate of several centimeters each year. This motion is known as plate tectonics, or more popularly as continental drift.
Exploration of the ocean floor has shown the existence of a number of midocean ridges that rise several
kilometers above the ocean floor and are thousands of kilometers long. They mark one type of plate boundary. For example, the Mid-Atlantic Ridge separates the North and South American plates from the Eu rasian and African plates, while the East Pacific Ridge separates the Pacific plate from the North American, Cocas, and Nazca plates. It appears that lava, forced upward from the asthenosphere into a midocean ridge, pushes out laterally from the ridge to form new plate material, which gradually cools, thickens, and solidifies at the trailing edge of the older plate material Rock samples from as far down as 8 kilometers below sea level verify that earth's youngest volcanic rocks are those found near these midocean ridges.
We have fu rther confi rmation that the plates move from the shape, geological structure, and fossil record of the continents. Still more evidence comes from rocks. Igneous rocks with similar magnetic fields, which were frozen at the time the rocks solidified, have been found at continental margins that are now widely separated.
Another line of evidence derives from the heat flow out of the earth's interior. Compared to the energy falling on the earth from the sun, the interior flow is scarcely a trickle: The heat conducted through an area the size of a football field is roughly equivalent to the energy given off by three 100-watt light bulbs. Yet over the 4.6-billion-year history of the earth this trickle of energy has contributed to the work of making continents drift, opening and closing ocean basins, building mountains, and causing earthquakes. The geographic variation in the heat flow from the interior is not great, but the global variation shows that the major oceanic ridges are high-heat-flow zones while the older conti nental shields and sedimentary regions are low-heat-flow zones.
How are the plates transported across the mantle?
It appears that they are driven by the horizontal flow of convective currents within the mantle, circulating in the upper, softer portion of the mantle. Often the leading edge of one plate is pushed downward and forced into the mantle, to create a deep trench. This process can form a coastal mountain belt, like the Andes, on the overriding plate. As the other plate descends over millions of years, it heats up and becomes part of the general circulation in the asthenosphere. Plates separate along midocean ridges. Most of the great geologic processes-volcanic activity, mountain building, formation of ocean trenches, earthquakes-are concentrated on or near plate boundaries.
